Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters, such as ladder filters having electrically connected series and shunt resonators formed in a ladder structure. The filters may be included in a duplexer, for example, connected between a single antenna and a receiver and a transmitter for respectively filtering received and transmitted signals.
Various types of filters use mechanical resonators, such as bulk acoustic wave (BAW), surface acoustic wave (SAW), and solidly mounted resonator (SMR)-BAW resonators. The resonators generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. A BAW resonator, for example, is an acoustic stack that generally includes a layer of piezoelectric material between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack and the thickness of each layer (e.g., piezoelectric layer and electrode layers). One type of BAW resonator uses an air cavity for acoustic isolation instead of being solidly mounted and may be referred to as a film bulk acoustic resonator (FBAR). FBARs, like other BAW devices, can be made to resonate at GHz frequencies, and are relatively compact, having thicknesses on the order of microns and length and width dimensions of hundreds of microns. This makes them well-suited to many applications in high-frequency communications.
Resonators may be used as band-pass filters with associated passbands providing ranges of frequencies permitted to pass through the filters. The passbands of the resonator filters tend to shift in response to environmental and operational factors, such as changes in temperature and/or incident power. For example, the passband of a resonator filter moves lower in frequency in response to rising temperature and higher incident power.
Cellular phones, in particular, are negatively affected by shifts in passband due to fluctuations in temperature and power. For example, a cellular phone includes power amplifiers (PAs) that must be able to deal with larger than expected insertion losses at the edges of the filter (duplexer). As the filter passband shifts down in frequency, e.g., due to rising temperature, the point of maximum absorption of power in the filter, which is designed to be above the passband, moves down into the frequency range of the FCC or government designated passband. At this point, the filter begins to absorb more power from the PA and heats up, causing the temperature to increase further. Thus, the filter passband shifts down in frequency more, bringing the maximum filter absorbing point even closer. This sets up a potential runaway situation, which is avoided only by the fact that the reflected power becomes large and the filter eventually settles at some high temperature.
PAs are designed specifically to handle the worst case power handling of the filter at the corner of the pass band. Currents of a typical PA can run from a few mA at the center of the filter passband to about 380 mA-450 mA at the edges. This is a huge power draw on the PA, as well as the battery that drives the cellular phone. This is one reason that a cellular phone operating more in the transmit mode (i.e., talk time) than in the receive mode (i.e., listening time) drains battery power more quickly.
In order to prevent or reduce frequency shift with rising temperatures, a conventional filter may include a layer of oxide material within the piezoelectric layer of the acoustic stack. The oxide material has a positive temperature coefficient of elastic modulus over a certain temperature range. The positive temperature coefficient of the oxide material at least partially offsets the negative temperature coefficients of the metal electrodes and the piezoelectric material, respectively. For example, the oxide material may be placed in the center of the piezoelectric layer or at either end of the piezoelectric layer between the electrodes. However, the acoustic coupling coefficient (kt2) of the resonator is greatly compromised by the addition of oxide material to the piezoelectric layer. This is because the oxide material appears as a “dead” capacitor in series with the active piezoelectric material dielectric.
Furthermore, the piezoelectric layer is often grown over the oxide material used for temperature compensation. The temperature compensation layer is generally an amorphous film and therefore is not an oriented crystalline material. As such, the piezoelectric layer grown on certain known temperature compensation layers (e.g., silicon dioxide) will not have a poor crystalline structure, and a random mixture of c-axis orientations in the thin film, which prevents good piezoelectric response. By contrast, it is desirable to form a highly textured C-axis piezoelectric material demonstrating excellent piezoelectric properties.
What is needed, therefore, is a temperature compensated acoustic resonator device that overcomes at least some of the noted shortcomings of known acoustic resonator devices described above.